The US is at a crossroads. Its future depends on the interwoven fates of two main groups: an ageing population of European extraction and a growing and mostly youthful population of Latin American ancestry. Unless they help each other out, warn leading demographers, the economic success that underpins the American dream may be under threat.

New Scientist's analyses of US Census Bureau data reveal large and stubbornly persistent disparities in wealth and educational achievement. If these are not narrowed, predicted population change could undermine the US's future prosperity. But the nation may yet avoid a cycle of decline – if it improves educational opportunities for young Hispanics.

At this year's Democrat and Republican conventions, the presidential candidates talked a lot about how voters were making a choice for a better future. They did not say much about the challenges posed by demography, however.

Perhaps they should have attended the annual meeting of the Population Association of America, held in San Francisco in May. The association's president, Daniel Lichter of Cornell University in Ithaca, New York, delivered a sobering address, warning: "Our failure to effectively address policy questions of persistent racial and ethnic economic inequality… may lead to new cultural and economic fragmentation."

Some researchers believe that Lichter paints an unduly pessimistic picture. But even the optimists agree that today's decisions will have far-reaching consequences. "We're really at a watershed moment in American society," says Richard Alba, a sociologist at the City University of New York.

Demographic transition is now under way (see diagram). In illustrating the US Census Bureau projections for 2050, New Scientist chose a scenario with relatively low rates of future immigration. Yet despite the political heat surrounding this issue, a similar picture will emerge whatever happens at the US border.

Immigration, especially from Mexico, has been the main driver of growth in the US Hispanic population so far. But net movement across the US-Mexico border is currently close to zero. Even if the US economy picks up, immigration will not return to its 1990s peak. Family planning policies mean that Mexico's birth rate has plummeted, so there will be fewer people leaving to find work.

With most groups in the US population reproducing at less than the rate needed to replace their numbers, higher fertility rates among Hispanics in the US should be sufficient to drive the transition towards a population increasingly characterised by young Hispanics and older, non-Hispanic whites.

A question of degrees

If US Hispanics enjoyed similar success to their counterparts of European extraction, there would be little to fear from this shift. But they don't. The US Census Bureau data analyses, run for New Scientist by Richard Fry of the Pew Research Center in Washington DC, reveal stark inequalities (see "Inequality in the US").

Hispanics earn less, on average, than whites, and the gap is even wider for accumulated household wealth, a key measure because it can protect against unemployment and other temporary reversals of fortune. This gap is likely to be exacerbated in the future by a concentration of wealth in the hands of older Americans – who will be predominately white.

Elsewhere in the world, such marked disparities might lead to unrest – but the lure of the American dream is expected to exert a calming influence. "One of the surprising characteristics of our society is the extent to which even highly disadvantaged, disenfranchised groups look towards the future of how they are going to 'make it'," says sociologist John Logan at Brown University in Providence, Rhode Island.

If more young Hispanics are to make it in the US, education will be key. Again, the numbers don't look good. Older immigrants with no higher education drag down the overall statistics on Hispanic educational attainment, so Fry compared US-born Hispanics with other groups, looking at the percentage of people in their late 20s with a college degree. Since the mid-1990s, there has been no narrowing of the gap between whites and Hispanics.

Findings like this leave Lichter worried. "There's a lot of built-in demographic momentum for future poverty and inequality," he says.

If he's right, that's bad news for the nation as a whole. The National Academies have warned that the US needs to invest heavily in science education if it is to retain the technological leadership on which future economic success may depend.

Demographic trends mean that the academies' recommended benchmarks are unlikely to be reached without an unprecedented rise in Hispanic students getting degrees in science, mathematics and engineering. "We need to triple or even quadruple the number," says Freeman Hrabowski, president of the University of Maryland, Baltimore County, and lead author of a 2011 National Academies report on expanding minority participation in science and technology.

While the numbers look bleak, some researchers take an optimistic view. "One has to be cautious about simply projecting the patterns of the recent past into the future," says Alba, who believes intermarriage between Hispanics and whites will narrow the gap between the two (see "How important is the melting pot?"). Alba also expects US universities to take steps to enrol more Hispanic students.

But that depends on elementary and high schools preparing Hispanic students for college in the first place. With those schools funded largely through local taxation, the issue becomes whether those who hold disproportionately more of the wealth, namely older whites, are willing to pay for the education of children from another ethnic group.

No one likes to pay taxes without seeing the benefit, so ageing whites may need to be convinced that investing in the education of young Hispanics is vital for their own future. There's a direct connection, argues Dowell Myers, a demographer at the University of Southern California in Los Angeles, as retirees not only need a vibrant and wealthy workforce to pay their retirement benefits, but also to buy their homes if it becomes time to sell.

So there's a deal that older white Americans could strike with young Hispanics: we'll pay for your education if you pay for our retirement. Myers believes this may need to be phrased simply in terms of old and young, so that the charged politics of race don't get in the way.

One encouraging sign is that the Democrat and Republican conventions both featured a prominent speech from a Hispanic politician. This suggests that the political elite is waking up to the fact that more of tomorrow's voters will be of Latin American extraction. The big question: will those voters get to share the American dream?

How important is the melting pot?

Intermarriage with the white majority, it is often said, is the final stage of assimilation into US society for a minority ethnic group.

Hispanics have a higher rate of such intermarriage than similarly disadvantaged African Americans. And some of the children of these marriages merge into the white population. Stephen Trejo at the University of Texas at Austin has found that only about two-thirds of 16- and 17-year-olds with one Mexican parent identify themselves as Mexican, according to the US Census question on Hispanic origin (Journal of Labor Economics, doi.org/dkrbpq).

Since the 1990s, however, the trend of increasing intermarriage between US Hispanics and whites has reversed. According to Daniel Lichter of Cornell University in Ithaca, New York, that seems to have been driven by second-generation US Hispanics reconnecting to their culture by marrying new immigrants (Sociological Forum, doi.org/b5dbv9). If so, reduced migration from Mexico could drive the rate of intermarriage up once more.

Still, Trejo suggests that the US should not rely on the melting pot of intermarriage to narrow the gaps in opportunity afflicting Hispanics. "It'll temper the problem a bit, but it's not going to solve it," he warns.

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Time to strike a deal (Image: William James Warren/Science Faction/Getty)